A dialog box with unknown file type is opened. The user double-clicks a file or selects Open from the file's context menu to open it. In some cases, a user may open a file, whose type is not given. When this occurs, the user is prompted by an “Open With” dialog box to enquire about the file. The user gets a short list of common file types in Windows XP.

Files that contain a particular type of data commonly have the same file name extension. It is appended to the file name and typically consists of a period followed by three alphanumeric characters, for example, ANSI text files have .txt file name extension.

For this, go to Start > Run menu, enter “regedit” and navigate to the registry path listed below. If the subkey “\Applications” is not present, create it as explained below. Then, create a string value name “NoOpenWith”, if this value does not exist. Now, right-click and modify the value data box with “Yes” to enable the setting and “No” to disable it.

Second, go to the [HKEY_CLASSES_ROOT\Applications\] key(the application you want to remove from the "Open With" dialog box, in this case it is Notepad), right-click on the blank area in the right pane and click on "New > String Value" to create a new String value with the name "NoOpenWith".

Third, right-click on the 'NoOpenWith' String value and click on "Modify".